The invention relates to a method and apparatus for evaluating physical properties of materials and, more particularly, for determining a physical property of a semi-conductive or electrically conductive material, such as a carbonaceous or other fiber, as a function of the relative capacitance of the material.
In the search for high performance materials, considerable interest has been focused upon carbon fibers. The term "carbon fibers" is used herein in its generic sense and includes graphite fibers as well as amorphous carbon fibers. Such carbonaceous fibers are electrically conductive and preferably contain at least about 90 per cent carbon by weight, and most preferably contain at least about 95 per cent carbon by weight. Graphite fibers exhibit a predominant x-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers, on the other hand, are defined as fibers in which the bulk of the fiber weight can be attributed to carbon and which exhibit an essentially amorphous x-ray diffraction pattern. Graphite fibers generally have a higher Young's modulus than do amorphous carbon fibers and in addition are more highly electrically and thermally conductive.
Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and carbon fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high tensile strength, and high modulus. Graphite is one of the very few known materials whose tensile strength increases with temperature. Uses for carbon fiber reinforced composites include recreational equipment such as gold club shafts, aerospace structural components, rocket motor casings, deep-submergence vessels, ablative materials for heat shields on re-entry vehicles, etc.
Carbon fibers, for optimum service, are required to meet predetermined criteria with respect to surface activity, bonding characteristics, and the like. For example, carbon fibers preferably exhibit a surface activity sufficient to form a strong bond with a resinous matrix material and thereby form a composite article exhibiting optimum physical properties, i.e., interlaminar shear strength.
In the past, the physical properties of carbon fibers have ordinarily been evaluated through the use of testing techniques which destroy or at least alter the fiber being tested and are extremely time consuming. For example, the bonding ability of a carbon fiber has been investigated by the evaluation of the horizontal short-beam shear strength of a test bar which incorporates the same within a matrix. This process is extremely laborious and time consuming, requires a skilled tester, and results in the destruction of the fiber. Likewise, the evaluation of a fiber surface area by the conventional BET (Brunauer-Emmet-Teller) method may require an average of about four hours of work by a skilled operator for each sample.
In addition to consuming great amounts of time of skilled personnel, these known testing methods are not readily adaptable to automation. Of course, the time requirements prevent the evaluation of fiber samples on a relatively continuing basis as the fiber is being manufactured. Moreover, the physical set-up required for these known tests makes these testing techniques impractical for use in relatively frequent quality control testing in a large scale carbon fiber manufacturing plant.
It is accordingly an object of the present invention to provide a novel method and apparatus for non-destructively evaluating a physical property of a material.
It is a more specific object of the present invention to provide a novel method and apparatus for non-destructively evaluating the surface characteristics of semi-conductive or electrically conductive fibers.
It is another object of the present invention to provide a novel method and apparatus for rapidly and accurately evaluating a physical property of a carbon fiber in relation to a corresponding physical property of a reference carbon fiber.
It is yet another object of the present invention to provide a novel method and apparatus for determining the relative capacitance of electrically conductive fibers to thereby provide an evaluation of relative physical characteristics of the fibers.
These and other objects and advantages are accomplished in accordance with the present invention as will become apparent to those skilled in the art from the following detailed description when read in conjunction with the attached drawings.